Among all transmission methods for optical transmission systems, the greatest receiver sensitivity can be achieved by a binary phase modulation (BPSK) and homodyne reception (intermediate frequency equals zero). For this reason and in spite of its complexity and the high requirements made on the stability and spectral clarity of the lasers, this method is particularly well suited for applications wherein the employment of optical amplifiers is not possible, and wherein the highest receiver sensitivity is particularly important, such as with long distance traffic networks with optical waveguides and for inter-satellite communications.
A further standard method for optical phase modulation is the utilization of the linear electro-optical effect (Pockels effect) in elements made of lithium metaniobate, which are realized either as a block (bulk modulator) or as integrated optical waveguide elements (integrated optical modulator, IOM).
The disadvantages of these methods are with the bulk modulator the narrow modulation bandwidth as well as the large power requirements, and with the waveguide modulator the high optical losses as well as the narrow upper limit of the optical output power.
However, large band widths, high optical output, good efficiency and small driver power are demanded for free space transmissions, for example inter-satellite connections, which cannot be achieved by means of the two previously mentioned modulator arrangements. The disadvantages of high optical damping and the narrow upper limit can be circumvented by means of an arrangement of transmitter oscillator-(laser) phase modulator-optical amplifier.
In this case the laser generates coherent light of low optical output which, following a loss-connected modulation, is brought to the required transmission output by means of a quantum-optical amplifier.
In essence, the following three principles are known for quantum-optical amplifiers:
in the fiber amplifier, a doped fiber is optically pumped and amplifies light during single passage, PA1 the bulk amplifier is a doped crystal, which is optically pumped and which amplifies light in the course of its repeated passage through the pumped zone, PA1 semiconductor amplifiers are electrically pumped and amplify light during the single passage through a pn-junction through which current flows.
The latter are extensively employed in fiber-optical communications technology, among other uses also as electro-optical switches following multiple branchings, as well as for the amplitude modulation of frequency-stable light, since, besides the latter, the direct amplitude modulation of the feed current also requires a frequency modulation.
In some system configurations this is used for data transmission by means of optical frequency keying. Since laser diodes consist of an optical cavity with specific resonance frequencies and of a semiconductor amplifier, this effect is achieved by changing the phase velocity of the light by means of the injection density in the semiconductor amplifier. Accordingly it is possible to produce a phase shift by means of the injection current during the single passage of the light through a semiconductor amplifier. However, at the same time a certain amplitude modification must be expected. Such methods were developed for fiber-bound communications systems (H. Dupont, M. J. Chawki, F. Tillerot, M. Thual and A. Poudoulec, "Measurement of the Effective Phase-Amplitude Coupling Factor alpha eff of High Speed Semiconductor Optical Amplifier Modulator", IEEE Photonics Technology Letters, vol. 6, No. 8, August, p. 942;
G. Grosskopf, R. Ludwig, R. Schnabel, H. G. Weber, "Characteristics of Semiconductor Laser Optical Amplifier as Phase Modulator", Electronics Letters, vol. 25, No. 17, Aug. 17, 1989, pp. 1187-1188;
Kun H. No, Richard J. Blackwell, Robert W. Herrick, Joseph L. Levy, "Monolithic Integration of an Amplifier and a Phase Modulator Fabricated in a GRINCHSOW Structure by Placing the Junction Below the Quantum Well", IEEE Photonics Technology Letters, vol. 5, No. 9, September 1993;
T. N. Nielsen, U. Gliese, J. Riishoj, K. Stubkjaer, P. Doussiere, P. Garabedian, F. Martin-Leblond, L. Lafragette, D. Leclerc, D. Fernier, "1 Gbit/s QPSK Optical Microwave Transmitter Based on a Semiconductor-Optical-Amplifier Phase Modulator and Phase-Locked DFB Lasers", OFC 94;
J. Riishoj, T. N. Nielsen and U. Gliese, "A 4 Gb/s 2-Level to 2 Gsymbol/s 4 Level Converter GaAs IC for Semiconductor Optical Amplifier QPSK Modulators MP", IEEE Journal of Solid-State Circuits, vol. 29, No. 10, October 1994, pp. 1277 to 1281).
In connection with this phase modulations were achieved in some embodiments even without generating a parasitic amplitude modulation. To achieve a wide modulation bandwidth, the actual semiconductor amplifier should have small dimensions, which requires low parasitic capacitors and therefore makes high threshold frequencies possible. With a given output density, low outputs of the modulated signal result, which does not meet the requirements in connection with optical transmissions in space. Instead, an output of approximately one Watt at a transmission rate of 1 Gbit/s should be available, wherein higher data rates require correspondingly higher optical output.